EN 253 205 (2/2562)Mobile Communication
Multiplexing & Multiple Access Techniques
in Mobile Communication
Asst. Prof. Nararat RuangchaijatuponElectrical EngineeringKhon Kaen University
Office: EN04325A, Email: [email protected]
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Multiplexing Techniques in Mobile Communication
• Coordinate the successful operation of multiple terminals over a wireless channel
• multiple simultaneous analog or digital signals are transmitted across a single data link.
• Originally for wired networks– Later adopted into wireless networks
• Assign a slot of time, a portion of frequency, a specific code voice
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Multiplexing Techniques in Mobile Communication (cont.)
• A communication system comprising three sources and destinations: (a) without multiplexing and (b) with multiplexing
Source: https://www.intechopen.com/books/multiplexing/overview-of-multiplexing-techniques-in-wireless-networks
Fixed-Assignment Access
• Voice-oriented networks
• Channel partitioning
• Predetermined basis
• Fixed allocation of channel resource– Frequency
– Time
– Code (spread spectrum code)
• FDMA, TDMA, CDMA, OFDMA
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Fixed-Assignment Access (cont.)
• Forward channel or downlink– Base station to mobile station
• Reverse channel or uplink– Mobile station to base station
• FDD vs. TDD– TDD open loop power control,
synchronization between downlink and uplink, low power (because of interference control), low complexity
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Frequency Division Multiple Access
• Transmit simultaneously
• Using separated frequency• 1G
• Concern– Adjacent channel interference (ACI)
• Forward link• Reverse link
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FDMA (cont.)
Source: https://www.minigranth.com/mobile-computing/multiplexing-mobile-computing/
FDMA (cont.)
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Source: https://www.intechopen.com/books/multiplexing/overview-of-multiplexing-techniques-in-wireless-networks
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FDMA (cont.)
FDMA (cont.)
• Adjacent Channel Interference– Forward link
• Adjust the sharpness of transmitter and receiver filters for separate carriers
– Reverse link• Near-far problem• (BS) difficult to detect the weaker signal• In order to handle near-far problem
– Frequency planning– Power control– Guard band
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FDMA (cont.)
• Advantages of FDM– This concept is applicable on both analog
signals as well as digital signals.
– Simultaneous signal transmission feature.
• Disadvantages of FDM– Less Flexibility.
– Bandwidth wastage is high and can be an issue.
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Time Division Multiple Access
• Time Division is used for a particular amount of time in which the whole spectrum is used.
• Time frames of same intervals are made such that the entire frequency spectrum can be accessed at that time frame.
Source: https://www.minigranth.com/mobile-computing/multiplexing-mobile-computing/
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Time Division Multiple Access (cont.)
• Fully digital format– Format flexibility
• Buffering, multiplexing function, time-slot assignment
• Capable to provide different access rates
• Need synchronization at the receiver
• 2G GSM– Improve voice quality, flexible to integrate data
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Time Division Multiple Access (cont.)
• Advantages of TDM– Single user at a time
– Less complex and more flexible architecture
• Disadvantages of TDM– Difficult to implement
• For Example : (Wired) ISDN(Integrated Service for Digital Network) telephonic service, (Mobile) GSM
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TDMA (cont.)
• 8-slot TDMA scheme used in GSM
• FDD• 124 carriers (FDMA)
in each direction• 13 kbps encoded
digital speech• Same as FDMA,
TDMA uses power control for near-far problem
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TDMA (cont.)
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TDMA (cont.)
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Synchronous vs. Asynchronous TDMA• Synchronous TDMA
• Asynchronous or Statistical TDMA
Source: https://www.intechopen.com/books/multiplexing/overview-of-multiplexing-techniques-in-wireless-networks
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Code Division Multiple Access
• Integration of various types of traffic
• Multiple users but no need for coordination
• Multiple users, same band, same time– Different codes
• Each user is a noise source to other users– Power control
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CDMA (cont.)
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CDMA (cont.)
• Also called spread-spectrum technique
• Advantages of CDM– Highly Efficient
– Less Inference
• Disadvantages of CDM– Less data transmission rates
– Complex in nature
• Example: Cell Phone Spectrum Technology (3G etc.)
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Capacity of CDMA
• Processing gain N• Information bandwidth
R• Transmission
bandwidth W• M simultaneous users• Received power from a
terminal P (equal, perfect power control)
• Received SNR for the target receiver Sr
RNW
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M
N
PM
NPSr
rr SR
W
SR
WM
11
1
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Practical Consideration
• Sectorization gain factor GA
– Usually equal to the number of sectors in the cell
• Voice activity factor GV
– Ratio of total connection time to active talk time
• Interference increase factor H0
– Interference from users in other cells
• Hence
0
11
1
H
GG
SR
W
SR
WM VA
rr
Performance Improvement
Factor
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Orthogonal Frequency Division Multiple Access (cont.)
• Multi-carrier modulation
• Efficient use of spectrum
• Data stream is distributed among multiple subcarriers separated closely and precisely
• In a simple OFDM-based system, the modulated signal are distributed over different subcarriers via Inverse Fast Fourier Transform (IFFT)
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OFDMA (cont.)
• Since the basis of the IFFT is unit vectors with equally angular separated in polar plane, the spectrum of OFDM signal is composed of N shifted sinc functions
an OFDM signal with four orthogonal subcarriers
Source: https://www.intechopen.com/books/multiplexing/overview-of-multiplexing-techniques-in-wireless-networks
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OFDMA (cont.)
• The subcarrier spacing is chosen such that the center frequency of each subcarrier is located on a null point of other subcarriers– No interference
• Example: IEEE WLAN standards, LTE/LTE-A
• Disadvantages– High peak-to-average power ratio (PAPR) impedes
proper performance of amplifiers at OFDM transceivers
– Sensitive to carrier frequency offset (CFO) -> cause inter-carrier interference (ICI).
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OFDMA (cont.)
• Advantages– Resilience in frequency selective environments
– Resilience in inter-symbol interference (ISI)• Lack of ISI also means simpler equalization mechanism and
reduction in hardware cost of the OFDM receiver
– Little affects by narrow-band interference• Narrow-band interference blurs the reference signals for
synchronization or corrupts the data
• Error correction codes and interleaving aid
– Spectral efficiency: closely separated frequency sub-channels yields higher spectral efficiency
– Low-computational complexity: FFT&IFFT modules are simple
Question & Discussion
Assignment
Resources• https://www.intechopen.com/books/multiplexing/overvie
w-of-multiplexing-techniques-in-wireless-networks
• https://www.minigranth.com/mobile-computing/multiplexing-mobile-computing/
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